ar71xx: fix minor syntax error in /lib/upgrade/platform.sh

Fix a '==' that should be a '=' in a test condition. Busybox fortunately
doesn't care.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

target/linux/ar71xx/base-files/lib/upgrade/platform.sh

@@ -111,7 +111,7 @@ tplink_pharos_check_image() {
 	# is accepted (loading the first 1.5M of a remote image for this check seems
 	# a bit extreme)
 	dd if="$1" bs=1 skip=1511432 count=1024 2>/dev/null | while read line; do
-		[ "$line" == "$model_string" ] && break
+		[ "$line" = "$model_string" ] && break
 	done || {
 		echo "Unsupported image (model not in support-list)"
 		return 1